GRB 230728A

C. Salvaggio (INAF-OAB), P. D'Avanzo (INAF-OAB), S. Laha (GSFC/UMBC),
D. M. Palmer (LANL), T. M. Parsotan (GSFC/UMBC/CRESSTII) and
M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 02:50:03 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 230728A (trigger=1181187).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 343.539, +28.151 which is 
   RA(J2000) = 22h 54m 09s
   Dec(J2000) = +28d 09' 03"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  Due to a telemetry gap, BAT lightcurve information
is not immediately available. 

The XRT began observing the field at 02:51:33.5 UT, 90.3 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source with an enhanced position: RA, Dec 343.51530,
28.17470 which is equivalent to:
   RA(J2000)  = 22h 54m 03.67s
   Dec(J2000) = +28d 10' 28.9"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 113 arcseconds from the BAT onboard position, within the
BAT error circle. This position may be improved as more data are
received; the latest position is available at
https://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (6.29 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 7.2
(+2.52/-2.21) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 7.13e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 99 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. 
The 8'x8' region for the list of sources generated on-board covers 100% of the
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.079. 

Burst Advocate for this burst is C. Salvaggio (chiara.salvaggio AT inaf.it). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)

D. Xu, S.Y. Fu, S.Q. Jiang, Z.P. Zhu (NAOC), Z. Gray (NOT), report:

We observed the field of GRB 230728A detected by Swift (Salvaggio et al., GCN 34282
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 ) using the the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations started at 04:26:55 UT on 2023-07-28, i.e., ~ 1.61 hr after the Swift/BAT trigger, and 3x300 s Sloan r-filter images and 5x200 s Sloan z-filter images were obtained, respectively.

No optical source is detected within or at the border of the Swift/XRT error circle (radius ~1.9 arcsec, 90% containment, Salvaggio et al., GCN 34282
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 ) down to upper limits of r > 24.2 mag (5-sigma) and z > 23.0 mag (5-sigma).

We note that there exists a galaxy south-western, ~3.7 arcsec away from the center of the Swift/XRT error circle. This galaxy is present in SDSS with r ~ 22.39 mag, z ~ 22.99 mag, z_phot ~ 0.34, and in Legacy Survey with r ~ 22.14 mag, z ~ 22.04 mag, z_phot ~ 0.37, respectively. Preliminary photometry of the galaxy from the NOT data gives r ~ 22.4 mag and z ~ 23.0 mag, which are consistent with the above archival values, indicating that there is no prominent optical afterglow in this potential host galaxy.

P.A. Evans, M.R. Goad, J.P. Osborne and A.P. Beardmore (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 1173 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 230728A, we find an astrometrically corrected X-ray
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 343.51572, +28.17433 which is equivalent
to:

RA (J2000): 22h 54m 3.77s
Dec (J2000): +28d 10' 27.6"

with an uncertainty of 2.1 arcsec (radius, 90% confidence).

This position may be improved as more data are received. The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).

This circular was automatically generated, and is an official product of the
Swift-XRT team.

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), B. Sbarufatti
(INAF-OAB), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAR), J. D. Gropp
(PSU), S. Dichiara (PSU), J.A. Kennea (PSU), A.P. Beardmore (U.
Leicester) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 6.7 ks of XRT data for GRB 230728A, from 86 s to 28.9
ks after the  BAT trigger. The data comprise 94 s in Windowed Timing
(WT) mode (the first 3 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The late-time light curve (from T0+5.6 ks) can be modelled with a
power-law decay with a decay index of alpha=1.19 (+0.20, -0.22).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.97 (+0.24, -0.23). The
best-fitting absorption column is  4.4 (+1.4, -1.2) x 10^21 cm^-2, in
excess of the Galactic value of 6.3 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.04 (+0.19, -0.18)
and a best-fitting absorption column of 7.0 (+1.5, -1.3) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 4.3 x 10^-11 (7.6 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     7.0 (+1.5, -1.3) x 10^21 cm^-2
Galactic foreground: 6.3 x 10^20 cm^-2
Excess significance: 8.1 sigma
Photon index:	     2.04 (+0.19, -0.18)

If the light curve continues to decay with a power-law decay index of
1.19, the count rate at T+24 hours will be 6.2 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.6 x
10^-13 (4.7 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01181187.

This circular is an official product of the Swift-XRT team.

S. R. Oates (U. Birmingham) and C. Salvaggio (INAF-OAB)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 230728A
100 s after the BAT trigger (Salvaggio et al., GCN Circ. 34282
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 ).
No optical afterglow consistent with the XRT position (Evans et al.,
GCN Circ. 34286
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 ) is detected in the initial UVOT exposures.

Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first
finding chart (FC) exposure and subsequent exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white_FC           100          250          147         >20.9
u_FC               312          562          246         >20.1
white              100         1361          372         >21.1
v                  641         5845          274         >19.4
b                  567         1338           78         >19.5
u                  312         1313          304         >20.0
w1                 691         1289           58         >18.5
m2                5851         6048          194         >19.5
w2                1023         1380           52         >18.5

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.080 in the direction of the burst
(Schlegel et al. 1998).

R. Strausbaugh (University of Minnesota), A. Cucchiara (NASA) report on
behalf of a larger collaboration:

We observed the GRB 230728A (Salvaggio et al., GCN 34282
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 ) field with the
LCOGT 1-meter Sinistro instrument at the Teide Observatory, Tenerife site,
on July 28, from 03:15 to 03:48 UT (corresponding to 0.42 to 0.97 hours
from the GRB trigger time) with the SDSS r and i filters.

We performed a series of 3x300s exposures in each band.  We do not detect
any source within the enhanced Swift-XRT error region (Evans et al., GCN
34286
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 ) in either band, consistent with later optical upper limits (Xu et
al., GCN 34285
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 ).

The following upper limits are calculated using the PanSTARRS catalog as
reference:

r > 23.3
i > 22.8

These magnitudes are not corrected for galactic extinction.

S. Dalessi (UAH) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 02:50:05.24 UT on 28 July 2023, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 230728A (trigger 712205410/230728118).
which was also detected by Swift BAT (Salvaggio et al. 2023, GCN 34282
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 ) and Swift XRT (Evans et al. 2023, GCN 34286
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 ).
The Fermi GBM on-ground location is consistent with the Swift XRT position.

The angle from the Fermi LAT boresight is 66 degrees.

The GBM light curve consists of multiple peaks with a duration (T90)
of about 19 s (50-300 keV). The time-averaged spectrum
from T0-6.1 to T0+12.3 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -1.2 +/- 0.2 and the cutoff energy,
parameterized as Epeak, is 95 +/- 15 keV.

The event fluence (10-1000 keV) in this time interval is
(1.7 +/- 0.1)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0-2.6 s in the 10-1000 keV band is 3.1 +/- 0.2 ph/s/cm^2.

A Band function fits the spectrum equally well
with Epeak= 78 +/- 20 keV, alpha = -1.1 +/- 0.3 and beta = -2.4 +/- 0.4.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html

For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"

Y.-D. Hu, E. Fernandez-Garcia, A. J. Castro-Tirado, M.D. Caballero-Garcia, R. Sanchez-Ramirez, I. Perez-Garcia and S. Guziy  (IAA-CSIC), C. Perez del Pulgar, A. Castellon, I. Carrasco and A. Reina (Univ. de Malaga) and F. Rendon (IAA-CSIC and INTA-CEDEA) on behalf of a larger collaboration, report:

Following the detection of GRB 230728A by Swift (Salvaggio et al., GCNC 34282), the 0.3m BOOTES-1B robotic telescope in Mazagon (Huelva, Spain) automatically responded to this burst on Jul. 28 at 02:50:54 UT (i.e. ~51 s after trigger). In the co-added frame (60 x 10 s, clear filter), no source is detected within the enhanced XRT position (Evans et al., GCNC 34286) down to 19.9 mag.

This non-detection is consistent with the upper limits reported by MASTER (Lipunov et al. GCNC 34281), NOT (Xu et al. GCNC 34285), UVOT (Oates et al. GCNC 34288) and LCOGT (Strausbaugh et al. GCNC 34289).

We thank the staff at INTA-CEDEA for their excellent support.

M. Stamatikos (OSU), S. D. Barthelmy (GSFC),

H. A. Krimm (NSF), S. Laha (GSFC/UMBC),

A. Y. Lien (U Tampa), C. B. Markwardt (GSFC),

D. M. Palmer (LANL), T. Parsotan (GSFC/UMBC),

T. Sakamoto (AGU), (i.e. the Swift-BAT team):



Using the data set from T-240 to T+962 sec from the recent telemetry downlink,

we report further analysis of BAT GRB 230728A (trigger #1181187)

(Salvaggio, et al., GCN Circ. 34282
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 ).  The BAT ground-calculated position is

RA, Dec = 343.514, 28.169 deg which is

   RA(J2000)  =  22h 54m 03.4s

   Dec(J2000) = +28d 10' 09.5"

with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment).

The partial coding was 49%.



He mask-weighted light curve shows a complex multi-peaked structure with some

emission prior to the trigger time.

T90 (15-350 keV) is 15.10 +- 0.89 sec (estimated error including systematics).



The time-averaged spectrum from T-5.62 to T+10.74 sec is best fit by a simple

power-law model.  The power law index of the time-averaged spectrum is

1.58 +- 0.13.  The fluence in the 15-150 keV band is 1.1 +- 0.1 x 10^-06 erg/cm2.

The 1-sec peak photon flux measured from T+0.41 sec in the 15-150 keV band

is 2.5 +- 0.3 ph/cm2/sec.  All the quoted errors are at the 90% confidence

level.



The results of the batgrbproduct analysis are available at

http://gcn.gsfc.nasa.gov/notices_s/1181187/BA/

Kiwanee' Smith (UVI), P. Gokuldass (ERAU), N. Orange (OrangeWave Innovative Science, LLC), D. Morris (NASA), K. Noonan (UVI), R. Querrard (UVI), T. Lombardi (Eckerd College), Dylon Smith (UVI)

We observed the field of GRB230728A (Salvaggio et al., GCN 34282
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 ; Evans et al., GCN 34286
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 ) with the 0.5m Virgin Island Robotic Telescope (VIRT) at the University of the Virgin Islands' Etelman Observatory on 07-28-2023 starting at 04:22:27.816 (T+1.3 hrs). We performed a series of exposures in R filter with a total exposure of 1560s. The weather conditions were clear during the hours of observation with an average airmass of ~1.2. 

We do not detect any source within the enhanced XRT position (Evans et al., GCNC 34286). This non-detection is consistent with the upper limits reported by MASTER (Lipunov et al. GCNC 34281), NOT (Xu et al. GCNC 34285), UVOT (Oates et al. GCNC 34288), LCOGT (Strausbaugh et al. GCNC 34289) and BOOTES-1 (Hu et al. GCNC 34291). We report the following 3-sigma upper limit: 

T_mid        ||Exposure ||Filter ||Limit 
T+ 1.5 hrs ||1560s         ||R        ||>20.5

The limit is estimated from comparison to nearby USNO B1 stars and is not corrected for Galactic extinction. The VIRT is still in the commissioning phase. 

We acknowledge financial support from NASA MUREP MIRO award 80NSSC21M0001, NASA EPSCoR award 80NSSC22M0063, and NSF EiR award 1901296. R.Q. and N.B.O. also acknowledge financial support from South Carolina Space Grant award 80NSSC20M0054. This message can be cited.